1. Field of the Invention
The present invention relates to a supercharging pressure control device for an exhaust gas turbocharger which supercharges the intake air to an internal combustion engine by driving a compressor together with an exhaust gas turbine in a rotating manner through use of the exhaust gas energy of the internal combustion engine.
2. Description of the Prior Art
Conventionally, an internal combustion engine with an exhaust gas turbocharger utilizes the energy of the exhaust gas to supercharge the intake air and supply it to the internal combustion engine. Consequently, the intake air charging efficiency is increased, which makes it possible to increase the fuel injection volume contributing greatly to increased output. However, if the supercharging pressure becomes excessive, an excessive stress is imposed on the internal combustion engine and on its intake and exhaust gas system with the attendant risk of damage or destruction. Accordingly, many means have been developed to prevent an excessive increase in supercharging pressure.
An example of one of these prevention means is a velocity control device for exhaust gas flow which is described in U.S. Pat. No. 2,944,786. In this device, a control valve is provided at the intake port section for exhaust gas in the exhaust gas turbocharger. By adjusting the degree of opening of the control valve, the velocity of the exhaust gas flow in the inlet port to the exhaust gas turbine is controlled, thereby preventing the excessive rotation of the exhaust gas turbine. Consequently an excessive increase in supercharging pressure otherwise caused by the compressor is generally prevented, while the exhaust gas flow velocity in a light or lower load range of operation is increased to increase the supercharging pressure and therefore the output in the lower load range.
Another example of such conventional prevention means is a bypass channel for connecting the upstream and downstream sides of the exhaust gas turbine, and the bypass channel is provided with a bypass control valve which opens when the supercharging pressure in the bypass channel on the downstream side of the compressor goes above a set value. When the supercharging pressure on the downstream side of the compressor exceeds a specified value, the bypass valve control device opens and the energy of the exhaust gas is exhausted to the outside through the bypass channel so that it will not act to rotate the exhaust gas turbine. This prevents excessive rotation of the exhaust gas turbine, thus preventing an excessive increase in the supercharging pressure.
The velocity control device for exhaust gas flow and the bypass valve control device can effectively carry out supercharger pressure control, but the following types of inconveniences still remain.
Specifically, with the velocity control device for exhaust gas flow, when the supercharging pressure exceeds the set value, the cross-sectional area of the exhaust gas inlet port is enlarged to decrease the flow velocity of the exhaust gas introduced into the exhaust gas turbine. But the full volume of exhaust gas is all introduced into the exhaust gas turbine and not bypassed, so that when the exhaust gas energy is further increased, the rotational speed of the exhaust gas turbine will go beyond control and the supercharging pressure is ultimately increased. In other words, a drawback of the velocity control valve is in insufficient flexibility of operation. Specifically, since the original design specifications of the exhaust gas turbocharger must ensure that the increase of supercharging pressure is below the allowable value, the overall torque characteristics must be generally set for a low supercharging pressure throughout engine operation, including the time that the engine is running under lower load. However, this results in a supercharging pressure in the lower load region which is unduly reduced, making it difficult to improve the output characteristics in this operating region.
Now turning to the bypass valve control device, when the supercharging pressure is about to exceed the set point, the exhaust gas is introduced into the bypass channel by the bypass control valve means, and is excluded from the region of rotation of the exhaust gas turbine. It therefore functions as an effective means of preventing an excessive increase in the supercharging pressure. However, the torque characteristics determined by the specifications of the exhaust gas turbocharger are constant, and simply act to cut excessive supercharging pressure, which does not solve the problem of low supercharging pressure in the low load region. It should be further noted that with such a bypass control device, immediately after the bypass control valve is opened, the exhaust gas turbine rotates at a relatively excessive speed due to the effect of its inertia, causing insufficient reduction of the supercharging pressure, resulting in the production of knocking. The back pressure is precipitously lowered, giving rise to the fear that the catalytic device in the exhaust system could be damaged.
In addition, it was found out that when either of these devices is used separately, if the degree of valve-opening remains small and operation becomes impossible because of a breakdown or seizure, the exhaust gas turbine and compressor rotate at an abnormally high rate and the supercharging pressure increases excessively, so that knocking could develop or engine damage could result.
Further, the energy of the exhaust gas may be large beyond the capability of these devices, which needs the fuel supply to stop.
Therefore, these prevention devices as mentioned above must be further improved, and development is still required in the gas intake system for supercharging in order to maintain flow flexibility and pressure variability throughout operation.